When attempting to collect an emission spectrum of a transparent fluid, particularly a gas, collection efficiency is critical due to the low degree of scattering by the sample. Such is the case in Raman and fluorescence detection and other forms of spectral analysis.
Existing collection geometries used in conjunction with transparent samples include in-line (forward scatter and backscatter), and 90-degree configurations. In a 90-degree configuration, illustrated in FIG. 1, an excitation beam 102 is roughly perpendicular to a collection path 104 within a zone 106 wherein sample scattering takes place. A first lens 108 is typically used to focus the excitation beam, and a second lens 110 is typically used to focus the collection beam onto a spectropraph entrance slit 112 or some other suitable detection means. Other elements such as filters and beam-directing elements have been eliminated from the drawing for the sake of clarity.
The 90-degree geometry offers the potential for high collection efficiency if the sample can be presented directly to the slit 112; for example, the excitation light is arranged to be parallel and coincident with the slit or imaged onto the slit. The configuration exhibits a serious drawback, however, in that the collection and excitation paths are physically separate, and thus use different optics. This leads to extreme sensitivity to alignment errors and inherent instability, which restricts the usefulness of this arrangement. The same holds true for forward-scatter configurations, wherein the excitation and collection are essentially co-axial, but oriented outwardly from a sample in opposite directions.
The use of backscatter, in contrast, is inherently stable because the excitation and collection paths may be combined so as to share the same optical path and associated optical elements. Referring to FIG. 2, some form of beam combiner 204 is typically used to merge or "fold" the excitation beam 206 into a combined excitation/collection path 210. The sample lens 212 in this case focuses the excitation energy into and collects spectrum from a small region 214. Optical filters (not shown) including the beam combiner 204 function to remove excitation wavelengths from the collection path, and a lens 216 or other elements including fiber optics may be used to relay the collected spectrum to a spectrograph input 220.
Thus, whereas the 90-degree geometry potentially collects data from the entire extended region along the slit, with backscatter data are collected only from the small region at the focus 214 of the sampling lens 210. As such, the collection efficiency of the backscatter arrangement is inferior as compared to the 90-degree geometry, leaving an outstanding need for a spectral analysis configuration having an improved collection efficiency without the need for critical optical alignment of separate excitation and collection paths.